DESCRIPTION (Verbatim from the applicant's abstract): Expansion of trinucleotide repeat (TNR) sequences is the causative mutation for a number of hereditary diseases, including myotonic dystrophy, the most common dystrophy in adults, Fragile X syndrome, the most common form of inherited mental retardation, and neurodegenerative diseases such as Huntington's and the spinocerebellar ataxias. The mechanism of TNR instability is interesting both for understanding the etiology and inheritance of the triplet repeat diseases, and for a basic understanding of genome stability in humans. In addition, expanded CGG/CCG and CTGICAG sequences are sites of chromosome fragility, areas prone to breakage in vivo. Chromosome breakage is implicated in the generation of translocations and deletions found in many types of cancer. The aim of this proposal is to elucidate the mechanisms involved in TNR instability and fragility, and determine how these two unusual characteristics are interrelated using Saccharomyces cerevisiae. A novel genetic assay has been developed that produces a selectable phenotype when a TNR tract expands or breaks. This assay will be used to screen for proteins whose over-expression influences TNR expansion or fragility. The proteins found to influence TNRs will be characterized to determine both their normal cellular functions and their influence on repeat maintenance. In addition, the hypothesis that TNR expansions occur by aberrant lagging strand replication will be tested by analyzing tract stability (by PCR) and fragility (by genetic and physical analysis) in specific yeast replication mutants. The role of the G2IM checkpoint in detecting TNR tract damage and preventing chromosome breakage will be investigated by comparing rates of TNR tract breakage in wild-type and cheokpoint-defective cells. Lastly, these analyses will be extended to other types of minisatellite sequences that act as fragile sites in human cells. The proposed experiments are designed to elucidate not only how simple repeats expand to cause human disease, but also the consequences of and cellular response to expanded tracts, with the goal of understanding how genomic instability can affect human health.